Carnitine is a component of human milk that is conditionally essential for the newborn infant. It is an obligate carrier for the transport of activated fatty acids across the inner mitochondrial membrane; and a modulator of the intramitochondrial pools of acyl- and free Coenzyme A (CoA). Infants receiving formulas enriched with medium-chain triglycerides (MCT) excrete medium-chain acylcarnitines suggesting that esterification with carnitine is a previously unrecognized mechanism for removing metabolites of dietary MCT. The overall aim of this project is to test the working hypothesis that administration of medium chain triglyceride (MCT) increases the nutritional requirement for carnitine. To test this hypothesis, the newborn piglet will be used to examine the effects of MCT on carnitine metabolism, as well as the effects of carnitine deprivation (carnitine depletion induced by carnitine-free parenteral nutrition) on the metabolism of exogenously administered MCT, and for comparison, long chain triglyceride (LCT) . Mechanisms of interaction between carnitine and fatty acid metabolism will be examined in vitro using cultured porcine hepatocytes. The project proposes to study the effect of MCT and LCT administration on carnitine balance, plasma and tissue ester pools (carnitine and CoA), clearance from plasma, urinary and biliary excretion, carnitine acetyltransferase (CAT) activity, hepatocellular uptake, and CAT induction. It furthermore proposes to study the effect of carnitine deprivation on fatty acid oxidation (metabolic plasma and tissue parameters after fat loading, 14 CO2-breath test), sequestration of CoA, altermative pathways of acyl group disposal (acylglycines, dicarboxylic acids), ketogenesis (ketone body kinetics), ketone body oxidation, fatty acid esterification and lipoprotein metabolism. These studies have both theoretical and practical significance. The characterization of an interaction between carnitine and MCFA metabolism would expand our basic knowledge and identify a novel nutrient-nutrient interaction. Practically, the widespread use of enteral and potential use of parenteral MCT in populations at risk for carnitine deficiency, e.g. premature babies and infants with malabsorption, underscores the importance of investigating potential interactions between carnitine and MCFA metabolism. Since carnitine esters generally reflect metabolic stress on the intracellular oxidative pathways, they may be useful indicators of MCT tolerance. More importantly, it should be clarified whether MCT administration is associated with an increased carnitine requirement.